It is known to imprint magnetic poles of alternating polarity on the surface of material by causing the material to travel in the immediate vicinity of the active portion of a magnetizing apparatus or in the air gap of such an apparatus producing an adequate magnetic field. The multipolar magnetization obtained on the material can be of the traversing or symmetrical type, which means that the two faces of the strip or of the sheet exert a magnetic attraction or pull strength of approximately the same value. On the other hand, it can be of a non-traversing or biased type and, in this case, one of the faces of the material exerts a biased or higher magnetic pull strength than the other face. The weaker or magnetically unbiased face may be advantageous for other uses and is able to receive, for example, some decoration, paint or an adhesive, or alternatively a sheet of mild magnetic material.
In order to magnetize a material, it is necessary to apply an adequate magnetic field to it, the intensity of which depends on the magnetic intrinsic coercive force of the material (and the direction of which depends on the field lines to be imprinted in this material). Typically the intensity of the applied flux field should be at least two times the intrinsic coercive force (Hci) of the material, and more particularly should be three or more, the general rule being that a magnetic field three times the value of the material Hci being necessary to achieve saturation magnetization.
In accordance with the prior art, magnetic fields are produced by direct, optionally pulsed electric currents by using, for example, electromagnets, coils (solenoids) or the discharge of capacitors.
These systems are essentially intended for single face magnetization. Nevertheless, they are expensive as they are complex, often fragile, subject to heating up and are high energy consumers and can be dangerous. They are limited in the number of poles per inch and in possible active surfaces due to problems of insulation of the conductors and the electromagnetic stresses applied to them. Moreover, the production rates are frequently limited to a strip speed of less than 3 m/min or about 10 ft/min, and even much less in the case of double face multipolar magnetization (i.e., transverse type).
Alternatively, in the prior art, the magnetic field may be produced by permanent magnets, in which case, the following benefits are obtained:
very low energy consumption limited to the mechanical energy needed for extracting the magnetized material from the apparatus, PA1 high reliability in operation, PA1 high safety in use due to absence of high voltage, the elimination of internal stresses in the apparatus. However, the main disadvantages of systems using Alnico or ferrite type permanent magnets are:
the production of a relatively weak magnetic field resulting in difficulty of effecting magnetization of moderately coercive materials, and the difficulty in obtaining the multipolar magnetization of magnetic materials in sheet form as described above.
One method of multipolar magnetization is set forth in U.S. Pat. No. 4,379,276 to Bouchara et al. which relates to a process and apparatus for permitting the magnetization of materials in the form of sheets or strips, such as magnetic rubber, wherein, a strip to be magnetized travels between stacks formed by a plurality of flat main magnets each of which is intermediate to flux conducting pole elements. The main magnets are magnetized through the thickness. The magnets and flux conducting pole elements are alternately stacked in axial alignment to form a cylindrical stacked array. The magnetic disks are aligned between the flux conducting pole elements and with the disks having opposing (i.e., mirror image) magnetizations with a flux conducting pole element located in between. Accordingly, a polar moment (North or South) is induced at the surface of the flux conducting pole element, and when two arrays are positioned with opposing opposite polar moments, the induced polar moment is enhanced. Preferably, these arrays have magnetic disks of equal thickness and flux conducting pole elements of equal thickness, and further the arrays are of the same size so that each induced pole has an opposing and opposite polar moment. It is believed that this configuration facilitates a flux circuit whereby the flux passes across a flux gap between the induced polar moments from the cylindrical surface of one array to the other in a first direction, through an adjacent magnetic disk in the direction of magnetization, across to the next array in a second direction opposite the first direction, and through a second magnetic disk to the first flux conducting element. When the pre-magnetized material is subjected to the flux circuits of the flux gap, the material is magnetized, i.e., one or more field lines are imprinted on the material. In the case of multipolar magnetization, the sample is polarized, i.e., is magnetized with alternating north and south poles.